1. Technical Field
The present disclosure relates to a common mode noise reduction circuit to reduce common mode noise generated, for example, from an image data communication interface between a display apparatus (e.g. liquid crystal display) and an image processor LSI. The present disclosure also relates to a differential signal transmitting apparatus including the common mode noise reduction circuit. The present disclosure further relates to a differential signal transmitting system including the differential signal transmitting apparatus, and a car electronics device including the differential signal transmitting system.
2. Description of Related Art
Along with the advance of the digital moving picture technology, high quality moving pictures have become available for mobile devices represented by smartphones, tablet devices, or the like. These kinds of small display devices that can display high quality moving pictures have started to be used as car electronics devices, such as car navigation systems, car entertainment apparatuses for playing movies or the like, and instrument panels to display information necessary for driving, such as speedometers, fuel gauges and odometers. Further, technologies that project an image in front of a wind shield so that drivers can obtain necessary information without looking away from their usual viewpoints, such as head-up displays or the like are being actively developed.
On the other hand, in order to realize these kinds of high quality image displays, a large amount of image data need to be transmitted to display devices. Accordingly, data communication bandwidth at interfaces between display devices and image processor LSIs has been steadily broadening. Due to the broadening communication bandwidth, electromagnetic radiation noise generated at interfaces between display devices and image processor LSIs has become a critical problem. Reduction of the electromagnetic radiation noise has become an important technical object for car electronics devices among others, and strict regulations have been implemented for safety.
Small amplitude differential interfaces represented by LVDS (Low Voltage Differential Swing) are widely employed for interfaces between display devices and image processor LSIs. It is widely known that transmitting signals through a pair of differential lines allows the magnetic fields generated from the differential lines to cancel out each other so that the adverse effect of the electromagnetic radiation is reduced.
As stated above, the differential mode component of the electromagnetic radiation can be canceled out. However, the common mode component of the electromagnetic radiation is not canceled out. Therefore, some electromagnetic radiation remains. Common mode voltages are controlled to maintain certain values. However, switching noise generated at switching when a transmitter generates data becomes common mode and appears as common mode noise. The switching frequencies and switching speeds of those transmitting apparatuses have increased as the communication bandwidth becomes wider, thereby increasing electromagnetic radiation noise.
A conventional common mode noise reduction circuit optimizes a control signal of a transistor depending on the polarization of the transistor that forms a switch, in order to reduce the switching noise. (For example, see Japanese Patent Application Laid-open No. 2012-105135.)